As is well known in the art, filters provide for the attenuation/rejection of signals with frequencies outside of a particular frequency range and little rejection/attenuation to signals with frequencies within a particular range of interest. These filters most typically take the form of blocks of ceramic material having one of more resonators/poles formed therein such as, for example, the ceramic filters disclosed in U.S. Pat. No. 4,431,977 to Sokola et al. and U.S. Pat. No. 4,692,726 to Green et al. A ceramic filter may be constructed to define either a lowpass filter, a bandpass filter or a highpass filter.
In a bandpass filter, the bandpass area is centered at a particular frequency and has a relatively narrow bandpass region, where little attenuation/rejection is applied to the signal.
The bandwidth of a filter can be designed for specific bandpass requirements. Typically, the tighter or narrower the bandpass, the higher the insertion loss, i.e., an important electrical parameter. A wider bandwidth, however, reduces a filter's ability to attenuate/reject unwanted frequencies, i.e., frequencies which are known in the art as rejection frequencies.
The use and application of a shunt zero such as, for example, the shunt zeros of the filters disclosed in FIG. 1 herein and, additionally, U.S. Pat. No. 5,502,422 to Newell et al. and U.S. Pat. No. 5,864,265 to Ballance et al. has been shown to improve the performance of filters by creating a notch or sharp point of increased rejection/attenuation as shown in FIG. 4 at a point close to the low side of the bandpass.
One disadvantage, however, which has been associated with the use of a single shunt zero is the increase in insertion loss and bandpass frequency ripple (e.g., the delta between the minimum and maximum points of a bandpass's insertion loss) as the rejection/attenuation moves closer and closer to the start and/or stop frequencies of the bandpass.
This disadvantage is of particular significance and consequence in repeater, micro cell and pico cell filter applications where high rejection and low bandpass ripple are two of the critical performance parameters.
Specifically, it is known in the art that repeaters, one of the intended applications of the filter of the present invention, are designed to eliminate reception problems in homes, office buildings, hotels, restaurants, etc. by amplifying the RF signal which is received before forwarding the RF signal either to a handset or base station. Most repeaters cascade filters in series with an amplifier therebetween to achieve the desired frequency rejection/attenuation. However, when filters are set up in series, high ripple and low rejection become a problem since lesser rejection causes distortion and excess ripple reduces the effective transmission distance of the repeater.
There thus remains a need for a filter designed to provide a high rejection/attenuation without a concomitant increase in ripple for repeater, micro cell and pico cell applications. The filter of the present invention meets these needs.